Oh boy, coccidiosis is a clear and present danger in all chicken yards. The problem is compounded by moist conditions. I would presume that you are having the very moist and wet spring that we are having here. Coccidiosis is everywhere and is much worse in warm and moist, as opposed to dry and hot, or wintertime when things are cold and frozen. There are many posts about this dreaded chick disease and you should read stuff to get a clear understanding of such. Basically, part of the issue is the birds' excrete, other birds peck at the ground, the oocysts (yes, called oocysts, where the cocci eggs are) are picked up by the other birds, ingested and the cycle begins. By the time you are seeing bloody stools, that is bad. The condition has been present and by this time is in advanced stage. I do not know a whole lot about this, but I know that bloody stools is just bad. I doubt that the bird brought there was the culprit, it was probably present already in your yard, it just progressed so. Possibly the bird brought the condition, but I would not totally blame it on that. I’m sure that others will chime in, perhaps my thoughts will be corroborated, perhaps not, but probably will. I am going to give you a couple of links to this disease, so you can read up about it.
Amprolium is the easiest drug to administer, it is 3-5 days on, if I am not mistaken, I would go 5 days personally. The sulfa meth is good too, but it is 2 days on the water drugs, four days off, 1 day on, four days off and then one day on again. A time consuming procedure, but this is how that drug works. Amprol is the easiest for surely. Remember, coccidiosis is everywhere, very similar to other illnesses, like marek’s disease.
https://wcps.canadian-forum.com/t1226-coccidiosis-needing-advice-and-feeling-a-little-emotional?highlight=coccidiosisIn this thread, please pay particular attention to post #5, Skeffling Lavender Farm, she has a good article on the disease. In my post, I had indicated my chicks had been on a two week program. Memory fails me and I have no clue why they were on a two week program, nor do I remember that that meant. I used amprol and it is only used for a few days. Anyways, please do some studying, it is a nasty thing and must be dealt with. Wishing you to keep the chin up and still have something to have an awesome day, CynthiaM.